How many concrete blocks do I need?
Divide the wall's face area by the face area of one block. Use the block's nominal size, which includes the 3/8 in mortar joint. A standard 8 × 8 × 16 in block is actually 7 5/8 × 7 5/8 × 15 5/8 in, so with mortar each one covers 8 × 16 in, or 0.889 sq ft.
Blocks = wall length (ft) × wall height (ft) × 1.125 + waste
A 40 ft wall, 4 ft high, is 160 sq ft. 160 × 1.125 = 180 blocks. Add 5% for breakage and cuts and buy 189. At 30 blocks per bag of masonry cement, that's 7 bags of mortar and about 1.5 tons of mason sand.
Subtract doors and windows in the per-wall field. Each wall is one rectangle, so add a gable or a stepped section as its own wall.
Block sizes and coverage
| Block | Nominal face | Blocks per sq ft | Blocks per 100 sq ft |
|---|---|---|---|
| 4, 6, 8, 10 or 12 × 8 × 16 in CMU | 8 × 16 in | 1.125 | 112.5 |
| 8 × 8 × 8 in half block | 8 × 8 in | 2.25 | 225 |
| Retaining wall block, 6 × 16 in face | 6 × 16 in | 1.5 | 150 |
| Retaining wall block, 4 × 12 in face | 4 × 12 in | 3 | 300 |
The first number in a CMU size is the wall thickness. A 6, 8 or 12 in block has the same 8 × 16 in face, so the count per square foot doesn't change. The weight, price and core fill do.
CMU block calculator: mortar, core fill and rebar
A common rule of thumb is one bag of Type S masonry cement for every 28 to 35 standard blocks, mixed with about 3 parts sand. The calculator uses 30. Premixed mortar mix already contains sand, so one bag lays fewer blocks; read the bag and change the number.
If your plans call for grouted cores, pick "Only cores with rebar" or "Every core" and the vertical rebar spacing. A fully grouted 8 in wall typically takes about 0.26 cubic feet of grout per square foot. Cores sit 8 in apart, so rebar every 48 in fills about 1 core in 6. The rebar count assumes one #4 bar per filled cell, as tall as the wall plus 2 ft of lap.
Bar size, spacing and bond beams come from your plans or local code. The calculator counts material for the layout you enter. It doesn't design the wall.
Retaining wall block calculator
Segmental retaining wall blocks are dry-stacked, so there's no mortar or core fill. Pick the face size that matches your block and enter the wall's length and full height, including the buried course. Manufacturers typically call for the first course to be partly buried and set on a compacted gravel base, with drainage gravel behind the wall. Follow the install guide for your block.
Taller walls, walls holding back a slope or a driveway, and walls near a property line often need an engineered design and a permit. Many codes draw the line at about 4 ft of retained soil. Check with your building department before you buy.
Block calculator for a wall with openings
Measure the full wall, then enter the area of each door and window in that wall's "Doors and windows" field. Keep the waste at 5% or raise it to 10% if the wall has many corners and cuts.
Pricing a block job
Masons usually price block work per block laid or per square foot of wall, with mortar, grout and rebar as separate lines. The labor default assumes about 12 standard blocks per labor hour for a mason and tender together. Set your own rate and production in contractor mode, add the footing from the concrete calculator, and send it as an estimate.
Questions
How many concrete blocks are in a square foot?
1.125 standard 8 x 16 in blocks per square foot, or 112.5 per 100 square feet, counting the 3/8 in mortar joint.
How many blocks does a bag of mortar lay?
A common rule of thumb is 28 to 35 standard blocks per bag of masonry cement mixed with sand. Premixed mortar mix lays fewer. Check the bag.
How much waste should I add for concrete block?
5% covers breakage and cuts on a simple wall. Use 10% for walls with many corners, openings or cut courses.
Is a cinder block the same as a concrete block?
In everyday use, yes. Most blocks sold today are concrete masonry units (CMU). The count is the same for any 8 x 16 in face.